The present invention relates to a method and apparatus for making a plastic container and an improvement thereof, and more particularly, to a method and apparatus for making tubular plastic containers whose at least sidewall portions are molecularly oriented and an improvement of such plastic containers.
Tubular plastic containers having molecularly oriented sidewall portions with a relatively small wall thickness (e.g., a thickness of from 0.1 to 0.3 mm) have improved container properties, such as transparency, gas-barrier properties, strength, shock resistance and so forth. However, in case of manufacturing a tubular plastic container of the type mentioned above having a large height-to-diameter ratio (e.g., from 1.5 to 3.0) by an integral molding method, when the sidewall portion is stretched in a solid phase so as to be molecularly oriented, the sidewall portion is undesirably locally thinned, so that it tends to be broken.
Moreover, in case of employing such a plastic container as a container for an acid juice or the like, after a heated juice (e.g., from 75.degree. to 95.degree. C.) is hot-packed, the plastic container is hermetically sealed with a top end portion, and then the hermetically sealed container is relatively rapidly water-cooled down near room temperature. A typical known plastic container of this type, however, has a sidewall portion which is cylindrical and has a practically uniform thickness along the circumferential direction. Therefore, the sidewall portion heated on hot packing thermally shrinks (since it has been molecularly oriented), and a reduction in the inside pressure due to cooling accompanies this, so that the sidewall portion is nonuniformly deformed, producing a local, permanent indentation, which will not recover even after opening. Since containers differ in the position and extent of the permanent indentation produced from each other, the commercial value thereof is remarkably lowered.
Although there are such plastic containers having a sidewall portion in the shape of a square tube with round corners, conventional containers of this type usually have corner portions extremely thin in wall thickness (e.g., about 0.1 mm). Therefore, there has been hitherto a problem that the above-mentioned large permanent indentations are undesirably produced in the corner portions of the plastic containers of this type, causing the commercial value thereof to be lowered.